In the area of high performance computing (HPC), such as high-end servers and supercomputers, improvements in the performance of signal processing have been increasingly desired in recent years. As a result, development of high-speed high-capacity (high-density) data transmission technique used in a transmission line between racks, between boards, within a board, and the like is a matter of urgent necessity.
However, with traditional electric transmission technology, which is reaching the limit of increasing the speed and capacity, it is generally assumed that data transmission performance (speed, bandwidth (=total throughput)) required by HPC systems is difficult to achieve.
As a breakthrough to such a limit in the electric transmission technology, optical interconnection technology of performing data transmission using light has begun to attract great attention. To actualize a high-speed high-capacity optical interconnection, the pitch of electric wiring of a semiconductor device (integrated circuit (IC)), such as a photoelectric conversion element, driver, or receiver amplifier, needs to be narrow. However, the limitations of processing accuracy restrict a reduction in the pitch of wiring on a printed board.
To address this, an intermediate substrate (interposer) for converting the pitch of wiring on a device into the pitch of wiring on a printed board is used. For high-speed high-capacity optical interconnection, with increasingly faster data speeds, it is necessary to reduce the capacity of an optical device to obtain a band of the optical device. Because of this, in particular, a photodiode (PD) may have a reduced diameter of its light sensing area, and that diameter may be smaller than the diameter of the core of the optical transmission line (for example, smaller than 50 μm in the case of a multi-mode fiber).
In this case, to couple optical signals that have propagated in the optical transmission line to a light sensing surface of the photodiode without a loss, a structure for concentrating the optical signals (condenser) is necessary. In consideration of mounting on an interposer, a gradient index (GRIN) lens which allows the flatness of the interposer is effective as the structure for concentrating optical signals from an optical transmission line.
FIG. 7 is a cross-sectional view of an optical transmission system 301 according to a first reference technique. As illustrated in FIG. 7, the optical transmission system 301 includes a gradient index lens 310, a substrate 320, and an electronic device 330.
The electronic device 330 is flip-chip mounted on the substrate 320 using a plurality of bump electrodes 332. The gradient index lens 310 is disposed inside the substrate 320. Although not illustrated, the gradient index lens 310 receives light that has exited from an optical transmission line and been reflected by a reflector, for example. The gradient index lens 310 has a refractive index with a parabolic variation that decreases from its central region to its outer region.
As illustrated in FIG. 7, when divergent light L exits from the gradient index lens 310, a component of the divergent light L that does not enter a photodiode 331 being a light sensing element of the electronic device 330 is lost. FIG. 8 is a cross-sectional view of an optical transmission system 401 according to a second reference technique.
As illustrated in FIG. 8, the optical transmission system 401 includes a gradient index lens 410, a substrate 420, an electronic device 430, an optical transmission line 440, and a reflector 450. The electronic device 430 is flip-chip mounted on the substrate 420 using a plurality of bump electrodes 432.
The gradient index lens 410 is disposed inside the substrate 420. The gradient index lens 410 has a refractive index with a parabolic variation that decreases from its central region to its outer region. The gradient index lens 410 receives light that has passed through a core portion 441 covered by a clad layer 442 of the optical transmission line 440 (optical path L11), exited from the core portion 441 (optical path L12), and been reflected by the reflector 450 (optical path L13).
Unlike the gradient index lens 310 illustrated in FIG. 7, the gradient index lens 410 concentrates incident light having an appropriate length (optical path L13) on a photodiode 431 of the electronic device 430 (optical path L14).
Japanese Unexamined Patent Application Publication Nos. 2006-330697, 2001-141965, and 2009-31633 are examples of related art.
For the optical transmission system 401 illustrated in FIG. 8, if the optical transmission line 440 becomes misaligned toward the direction remote from the reflector 450 (optical transmission line 440A), for example, divergent light exiting from the optical transmission line 440A (optical path L12A) and light reflected by the reflector 450 (optical path L13A) also become misaligned.
In response to this, the location of light concentrated by the gradient index lens 410 (optical path L14A) is away from the photodiode 431. When the location of concentrated light deviates, as described above, an optical coupling loss occurs. Such an optical coupling loss occurs if the electronic device 430 is used at not only the light sensing side but also the light emitting side.